For MEMS (microelectromechanical systems) and other sensor applications and packages, a dedicated link (port) is usually provided between the outside environment and the sensor structure. Most sensor structures are highly sensitive to particles and contamination. Hence, the port should provide protection against particles and contamination. Most common MEMS packages can be related to two basic types: open cavity packages OCP and molded cavity packages MCP.
In OCP packages, a cavity is formed mostly by a cap/lid which is fixed on a substrate. The cavity is typically formed by a pre-molded substrate or special lamination technique. Different cap/lid types and materials are available such as metal caps or molded lids. Also, several substrates are available such as PCBs (printed circuit boards), leadframes or pre-molded multi-layer substrates. The cap/lid is attached to the substrate typically by gluing or soldering. The cavity provides space for the sensor die and additional devices such as an ASIC (applicant-specific integrated circuit). For electrical connection, wire bonding as well as flip-chip technologies are typically used. If the inner volume (cavity) of the OCP package is to remain in contact with the outside environment, e.g. for sensor applications, a port is formed in the cap/lid or in the substrate for this purpose. Often such a port is implemented via a small vent hole in the cap/lid. The dies in the cavity are protected against mechanical loads, but not against particles, corrosive gases or fluids with such a configuration.
An MCP package is an over-molded package type. Leadframes or PCBs are commonly used as substrates. For electrical connection, wire bonding is commonly used. After die attachment and bonding, the package is overmolded. For this purpose, a special molding technology referred to as film assisted molding (FAM) is used. FAM allows for the formation of an opening in the mold compound. This opening then represents the port for sensor structures on the sensor die.
Both OCP and MCP package types fail to provide particle protection directly. Additional effort is needed to provide particle protection such as special meshes applied on the port which, increasing the overall package cost. Furthermore, both OCP and MCP package types are limited in minimum package footprint and height. This again increases overall package cost.
In the case of OCP packages, the footprint is limited due to three factors: the additional space consuming bonding area for the cap/lid; the wall thickness of the cap/lid; and the clearance needed between the dies and the cap/lid. This requires around about 700 μm of additional space in each dimension. In case of wire bonding, landing pads also increase the package dimensions. Moreover, the package price for OCP type packages is strongly driven by the cap/lid itself and by the cap/lid attachment which causes additional process steps such as glue dispensing and cap/lid placement. Such processes are mostly serial and time consuming.
The FAM process used for MCP type packages is a highly complex process and requires conformity with several often-conflicting specifications. Hence, the minimum port or opening size is limited due to many factors, including substrate tolerance, die attachment tolerance, mold tool tolerance and positioning tolerance of the substrate strip in the mold tool. Additional influencing factors such as mold pressure and mold tool force on the sensor structures must also be considered. As a result, the benefit in size reduction of MCP type packages compared to OCP type packages is minimal, and because of expensive mold tools and machine equipment, the resulting cost reduction is trivial.
The disadvantages listed above for standard OCP and MCP package types become much more crucial if monolithically integrated sensor-ASIC dies are used. In this case, the percentage between package size and die size decreases strongly.
As such, there is a need for a more cost effective sensor package with a port.